A modern internal combustion engine is able to encompass different operating modes—so-called combined combustion processes—and different operating strategies. Thus an appropriately equipped spark-ignition engine is able to be operated, for example, in a stratified mode or in a homogenous mode. A CAI-capable (Controlled Auto Ignition) spark-ignition engine is able to be operated in specific operating ranges in a (controlled) self-ignition mode. Outside these operating ranges, an air-fuel mixture of the spark-ignition engine is spark-ignited in the conventional manner in so-called SI mode (spark ignition). An HCCI-capable (Homogenous Charge Compression Ignition) diesel engine permits, in a similar manner to the CAI-capable spark-ignition engine, combustion of a homogenous air-fuel mixture, the combustion starting almost simultaneously in the entire combustion chamber. An internal combustion engine with a variable valve train permits different operating strategies and/or modes for the purposes of final throttling, for example so-called early closing of the inlet (EIC mode: Early Inlet Closing) or so-called late closing of the inlet (LIC mode: Late Inlet Closing).
Transferring between the modes is technically challenging, in particular when a digital valve lift changeover is connected thereto. This is the case, for example, when transferring from SI mode to CAI mode of the internal combustion engine, as a CAI-combustion method for igniting a homogenous, lean air-fuel mixture requires a large quantity of residual gas which is produced, for example, by retaining exhaust gas from a previous combustion cycle. This is achieved by means of a corresponding profile of an outlet camshaft with a suitable valve lift and suitable phase position. In this connection, a changeover to a small valve lift is carried out on the outlet side, by which residual gas remains in a relevant combustion chamber of the internal combustion engine. It is also possible to conduct the residual gas by exhaust gas recirculation into the relevant combustion chamber of the internal combustion engine.
For reasons of comfort, a transfer between modes which is smooth and thus neutral in terms of torque is desirable. In other words, the effective engine torque has to be kept constant before and after the transfer between modes. When transferring between an SI mode and a CAI mode, this is achieved as follows: in order to achieve a pressure level in an intake manifold which is desirable for the CAI mode, a throttle flap is opened (final throttling). At the same time, an ignition angle is retarded in order, in the case of a constant air-fuel ratio, to reduce engine torque which is generated due to extra inflowing air and fuel which has been introduced (EIC mode). The retardation of the ignition leads to a marked increase in the exhaust gas temperature. With a changeover from the conventional SI mode simply into EIC mode, which is also spark-ignited, such a method is entirely practicable. However, should the mode be transferred to the CAI mode, the exhaust gas temperature has to be reduced before the transfer, as otherwise a subsequent CAI-combustion takes place too early, which results from a high charge temperature. The high charge temperature occurs due to the mixing of the hot, retained residual gases, a quantity of fresh air which has been drawn in and a quantity of fuel which has been introduced.
Premature ignition of the air-fuel mixture leads to knocking of the internal combustion engine, which has to be avoided at all costs to protect the internal combustion engine. Noise emissions are also produced by the knocking. Remedies are provided by less fuel being injected in EIC mode and the ignition angle being advanced to compensate for the torque. As a result, the air-fuel mixture shifts in the lean direction. This leads, however, to an increase in the nitrogen oxides produced during combustion. As CAI-combustion engines generally do not have an SCR-catalytic converter but only a conventional three-way catalytic converter, each transfer from the SI mode to the CAI mode brings the drawback of (nitrogen oxide) emissions. The emissions behavior of the internal combustion engine is thus impaired over the long term. Moreover, a transfer between the CAI mode and the SI mode, results in negative emissions behavior of the internal combustion engine as, for stoichiometric operation (SI mode) of the internal combustion engine, a lean residual gas of the CAI mode is produced and has to be removed from the relevant combustion chambers.